Multipole circuit interrupter

ABSTRACT

A multipole circuit interrupter, having an elongated metal-core tie bar supporting the plurality of movable contact arm structures, comprises improved means supporting the tie bar for pivotal movement with insulating means preventing a phase-tophase fault through the metal core.

' United States Patent.

Inventors Eugene J. Walker; [56] References Cited Donald G. Portman, both of Beaver, P8. UNITED STATES PATENTS P 3,418,607 12/1968 Walker e161. 335/41 Flled Sept. 17, 1969 3,336,545 8/1967 Stokes 335/41, Patented Aug. 17,1971

Assignee wesfinghouse Electric cbrporafion 3,193,643 7/1965 Hollyday 335/41 X ,Pmsbur h Pa 3,003,046 10/1961 De'l'orre.. 200/153 (.7) g 2,759,063 8/1956 Rypinski 335/41 x Primary Examiner-Milton O. Hirshfield Assistant Examiner-B. A. Reynolds AIIOI'IIB)ISA. Stratton, McHale and 10 Claims, 6 Drawing Figs. 7 I [1.8. CI 200/153 G, ABSTRACT: A multipole circuit interrupter, having an elon- 1 335/41 gated metal-core tie bar supporting the plurality of movable Int. Cl H0lh 9/24 contact arm structures, comprises improved means supporting Field of Search..... 335/41; the tie bar for pivotal movement with insulating means 200/153, 153 C preventing a phase-to-phase fault through the metal core.

.3. 1. .l l 1 L PAIENIEI] AUG! 7 I97! SHEET 1 [IF 2 FIG.I.

WITNESSES Wad W MULTIPOLE'CIRCUIT rnmimur'rsk BACKGROUND'OF THE INVENTION 1. Field of the Invention This invention relates to multipole circuit interruptersof'the type comprising-a metal-core tie bar supporting a plurality of movable'contact arms structuresfor simultaneousmovement.

between open and closed positions. In this type of interrupter, it is necessary to insulate at least one end of the metal core from the associated metal'support plate in order to avoid a phase-to-phase fault from the one outer pole-unit contact arm structure to. the one metal core then through'themetal coreto the other metal support plate to the: other outerpole-unit contact arm structure. For some applications, it is necessary to insulate both ends of themetal'core' from the associated metal support plates for adequate'protection against .phase-to-phas fault between the outer pole units.

2. Description of the Prior Art One prior art way of supporting the metal-core tie bar'and insulating the interrupter to avoid the abovementioned phase-to-phase fault throtighthe metal core is to drill holes in the ends of the metal core and cement insulating pivotsupport pins in those holes. The insulatingpivot support pins are then pivotally supported on two outer metalsupport plates in" openings in the support plates, and insulating washers are mounted on the insulating pinsbetween the metal core and the sidewalls of the metal support plates.

SUMMARY OF THE INVENTION Themovable: contact arm structuresof a rnultipole insulatand an insulatingv cover on the' core. The metal core'is movable contact23-is part of a movable contact arm structure machines at the opposite ends thereof to provide a pair of n metal-support pins..A pair oflflat insulatingbearingmembers are mounted onthe pins, and theassemblyisdropp'edinto the baseof the housing with the pinsmovingzdownin sidewall slots provided in the housingbase. Eachof the insulating bearings engages the associated-housinggsidewall at one 'side thereof and the steel core .at the other side thereo'f tosupport the tie bar against lateral movemenuA' pair of-flat metal support plates are dropped in" thehousing along the sidewalls of the housing over the insulating bearings with an undersurface of eachof the plates engaging the front surface of theassociated insulating bearing to support the associated insulating bearingv and tiebar against upward movement. The insulating bearings, and tie bar, are supported: against backward movement by. the engagement of the back surfaces of the insulating bearings with support surfaces on the housing. base. Each of the metal support plates comprises a pair of legsthat are moved through associated openings in the bottom of the base andspun over orriveted over to fix the support plate on the base. Thus, the tie bar, which is readily assembled in the housing base, is supported onthe insulating bearings for pivotal movement about-the elongated axis thereof-withthe' insulating bearings-providing adequate insulation to prevent aphase-tophase fault between theouter pole units through: themetal core.

BRIEF DESCRIPTION OF- TI-IE DRAWINGS FIG. 1 is a top plan view, withparts broken away, of a threepole circuit breaker constructed inaccordance with principles of the invention;

FIG. 2 is a side sectional view through the center pole unit taken generally along the line lI-II ofFIG. 1; l

FIG. 3 is an encl'view with partsibrokenaway of the tiebar and parts of the insulating base; 1

FIG. 4 is a partial sectional view taken generally along the line lV-IV of FIG. 3;

. FIG. 5 is a partial sectional view takengenerally along the line V\ of FIG. 3; and

FIG. 6 is a partial view illustrating the end ofthe tie bar and one of -the insulating bearings before the bearing is moved onto the tiebar.

DESCRIPTION'OF THE PREFERRED EMBODIMENT Referringto the drawings, there is-shown,.in FIGS. 1 and 2, a-multipole'molded-case-typecircuit breaker S'comprising an insulating: housing 7 and-a circuit breakerstructure 9 supported-in the housing 7. Except for the means for supporting and insulating the'tie bar,the' circuit breaker structure 9 is of thetypethatis-more specifically described in the US. Fat. to E. J. Walker et al. No. 3,418,607which discloses a two-pole circuit breaker as opposed to the three-pole circuit breaker shown in FIGS. 1 and 2. Thus, only a brief description of the circuit breaker structure 9'is herein provided.

Theinsulating housing 7-is a two-part housingcomprising a molded insulating base 11 and a molded'insulating cover 13 securedto the base 11. A pair of insulating barriers 15 (only one'beingseen in FIG. 2) molded integral with the insulating cover"l 3, cooperate with a pair of insulating base 11, to separate' the housing into three adjacent internal pole-unit compartments forhousing thethree pole units of the circuit breaker:

Each pole of the circuit breaker includes a stationary contactl9and a'movable contact 23. The stationary contact 19 is mounted on'the inner end of a conductor ,25 that extends into anopencavity in theoneend-of the housing where a solderless terminal conn'ector 28*is secured to the conductor 25 to enable connection of the pole unit in an electric circuit. The

theincludes a switch arm 27, a contact arm29 having the contact 23 secured thereto, and a contact arm 29 having the contact 23 securedthereto, a'nda contact-pressure spring 31. The-switcharmQTis fixedly secured to a common tie bar 33. The contactarm is pivotally supported on' the switch arm 27 by means'of an: inverted'U-shaped strap 35 (FIG. 4) that is secured to the contact arm 29 and that is provided with openings in the opposite legs thereof for receiving a pin 37. The'switch arm 27 is a generally U-shaped member comprising a pair'of sidewalls with-the pin 37 being supported thereon between the sidewallsof the switch arm. A pin 39 (FIG. 4) is supported between the sidewalls of the switch arm 27 for limiting clockwise movement of the switch arm 29 about the pivot 37.' Acontact-pressure spring 31 is supported between the bight portion of the generally U-shaped switch arm 27 and the undersurface of the contact arm 29-to bias the contact arm 29in a clockwise direction about the pin 37 to thereby pro- .videcontact pressure between the contacts in'the closed position'of the contact arm structure. In each pole unit, a flexible conductor 41 is connected at one end thereof to the associated contact 41 is connected at one end thereof the the associated contactarm 29 and at the other end thereof to a conductor'43 that is in turn secured to a looped conductor 45 that is in turn securedto a conductor 47 that extends into a cavity atthe outer end of the circuit breaker where a solderless terminal connector 49 is secured to the conductor 47 in order to permitconnection of the pole'unit in an electric circuit. As can be understood with reference to FIG. 2, the circuit through each pole unit extends from the terminal connector 49-through the conductor 47, conductor 45, conductor 43, flexible conductor 41, contact arm 29, :movable contact 23, stationary contact 19, conductor 25 to the other solderless terminal connector 27.

The contact arm structures 25 for the three-pole units are simultaneously operated between the open and closed positions by operation of a single operating mechanism 51 that is located in the center pole-unit compartment. The operating mechanism 51 is supported on a pair of supporting plates 53 (only oneof which is seenin FIG. 2) that are supported in the center pole-unit compartment. An inverted generally U- shaped operating member 55 is pivotally supported at the inner ends ofthe legs thereof in U- shaped notches in the frame or plates 53. A releasable tripmembei 57 is pivotally supported on the plates 53 by means of a pin 59. One end of the releasable trip member 57 is' latched in the operating position seen in FIG. 2 by means ofa latch mechanism 61. A pair of toggle links 63 and 65 are pivotally connected together by means of a knee pivot 67. The upper toggle link 63 is pivotally connected to the releasable trip member 57 by means of a pivot pin 69 and the lower toggle link 65 is pivotally connected to the switch arm 27 of the center pole unit at the pivot pin 37. A pair of tension springs 73 (only one 'of which is seen in FIG. 2) are supported under tension between the knee pivot 67 and the bight portion of the operating member 55. An insulating member 75, which is connected to the front portion of the operating member 55, comprises a handle part 77 that protrudes out through an opening 79 in the housing cover to permit manual operation of the circuit breaker.

The tie bar 33 is supported for pivotal movement about the elongated axis thereof in a manner tobe hereinafter-more specifically described. In order to manually open the circuit breaker, the handle 77 is moved from the closed position seen in FIG. 2, in a counterclockwise direction about the pivot at the inner legs of the operating member 55, tothe open psition. During this movement, the line of action of-the overcenter springs 73 is moved to the left .tocollapse the toggle 63, 65 to thereby move the contact arm structure 25 of the center pole unit to the open position which movement, because all three of the contact arm structures 25 are fixed to the tie bar 33, pivots the tie bar 33 to simultaneously move all three contact arm structures 25 to the open position. The contact arm structures 25 are stopped in the open position by engagement of the center-pole contact arm 29 with a stop pin 81 that is supported between the center-pole supporting plate 53. The circuit breaker is manually closed by moving the operating member 55 in a clockwise direction which'movement moves the line of action of the tension spring 73 to the right to erect the toggle 63, 65 thereby moving the contact arm structure 25 for the center-pole unit to the closed position seen in FIG. 2 which movement, because all three of the contact arm structures 25'are fixedly secured to the common tie bar 33, pivots the tie bar 33 to simultaneously move all three contact arm structures 25 to the closed position. I

The latch structure 61 (FIG. 2) is latched by means of a common insulating trip bar 83 that is operated upon the occurrence of an overload in any of the pole units to release the latch structure 61 to thereby release the trip member 57 toeffeet a tripping operation of the circuit breaker. In each pole unit, there is a thermal and magnetic trip device 85 that is operated upon the occurrence of overload currents to move the trip bar 83 to the tripped position. The operation of the trip device 85 and the tripping operation of the circuit breaker, along with the resetting operation following a tripping operation, is more specifically described in the abovementioned patent to E. J. Walker et al. U.S. Pat: No. 3,418,607. When the trip member 57 is released, the tension springs 73 pivot the trip member 57 in a clockwise direction and the springs collapse the toggle 63, 65 to move the contact arm structures 25 to the open position. Following a tripping operation, the circuit breaker is reset by movement of the bandle 77 to a position slightly past the full off position during which movement a shoulder portion 87 of the operating member 55 engages a shoulder portion 89 on the trip member 57 to rotate the trip member 57 in a counterclockwise direction, and the trip member 57 is relatched in a well-known manner by the latch structure 61 at the end of this movement. Thereafter, theoperating handle ,77 can be released and the circuit breaker can be operated in the same manner as was hereinbefore described.

The trip bar 83 and tie bar 33 both extend transversely across the circuit breaker into all three pole-unit compartments. The insulating barriers 17 on the base 11 are provided with slots or openings therein to permit mounting of the trip bar 83 and tie bar 33 extending through the openings in the barriers 17 to reach all three compartments. During the assembly of the breaker, when the housing cover 13 is removed,

the .trip bar 83 and tie bar 33 are dropped down into the base l1 into the mounted position on the base. The openings in the barriers 17 are then closed by suitable interlocking barriers.

When the assembly of the complete mechanism on the base is bar 33 in the circuit breaker housing for providing adequate insulation to prevent a phase-to-phase fault through the metal core of the tie bar 33. Referring to FIGS. 2 4, the tie bar 33 comprises a rigid steel core 101 and an insulating sleeve cover 103 on the steel core 101.The steel core 101 is machined at the opposite ends thereof to form steel pivot pin parts 105 at the opposite ends of the steel core. The outer sidewalls 107 (FIG. 3) of the insulating base 11 are formed with slots 109 (FIGS. 2, 3 and 5) therein which slots are open at the top of the open base 11. The top of the housing base 11 is illustrated at the line 108 (FIGS. 2 and 3) where the cover 13 engages the base 11'. A pair of flatrigid insulating bearings 113, each of which has an opening 115 (FIG. 5) therein for receiving the associated pin 105 to support the pin 105 for pivotal movement therein, are mounted onthe support pins 105 at the opposite ends of the tie bar 33. During the assembly operation, when the housing cover 13 is removed, the tie bar 33, with the movable contact structures 25 mounted thereon and with the insulating bearings 113 mounted on the pivot support pins 105, is dropped down into the base ll through the open top of the base 11, during which movement, the pins 105 move down into the slots 109 which are open at the top of the base 11. As can be understood with reference to FIG. 3, each of the insulating bearing members 113 engages the associated inner sidewall 119 of the associated housing sidewall 107 at one flat face thereof and the surface area 121 of the steel core at the other flat side thereof to support the tie bar 33 against lateral, or axial, movement. The surface area 121 (FIG. 3) of the steel coreis that surface, area that is left at the end of the steel core after machining the core to provide the pivot pin parts 105. Each of the insulating bearing members 1 13 engages a bottom surface 123 (FIGS. 3.,and 5) of the insulating base 11 to support the tie bar 33 against downward movement. A pair of flat rigid metal support plates 125 are then dropped down into position on the base and secured to the base to support the bearings 113 against upward movement inthe housing. As can be best understood with reference to FIG. 5, each of the support plates 125 comprises an' annular undersurface (a thickness or end surface) 127 that engages and mates with an annular front surface (a thickness or end surface) 129 of the associated insulating bearing 1 13 to limit upward or frontward movement of the insulating bearing 113. Each of the support plates 125 comprises a pair of legs 131 that extend through openings 133 in the bottom of the base 11, with the outer ends of the legs 131 being riveted over or spun over to fixedly secure each supporting plate 125 in position on the housing base 11. As can be understood with reference to FIG. 5, the insulating bearing member 113 provides insulation between the periphery of the metal support pin 105 and the surface 127 of the metal support plate 125. Each insulating bearing member 113 engages a surface 123 of the housing base 11 to support the bearing 113 and tie bar 33 against backward or downward movement, and the surface 127 of the associated support plate to support the bearing 113 and tie bar 33 against frontward or upward movementjAs was hereinbefore set forth, each insulating bearing member 113 engages the surface 119 of the associated sidewall 107 of the insulating base 11 at one side thereof and the surface 121 (FIG. 3) of the tie bar 33 at the other side thereof to support the tie bar 33 against lateral or axial (FIG. 3) movement. In the closed position of the contacts (FIG. 4) the contact-pressure spring 31 is compressed between the contact arm 25 and switch arm 27 to exert a downward force on the tie-bar 33 with the insulating bearings 113 engaging the surfaces 123 of the base 11 to support the insulating bearings-113 and tie bar 33 against downward movement. The circuitbreaker is shown in the closed position in FIG. 2. As was hereinbefore set forth, when the circuit breaker is operated to either the open or the tripped positions, the springs 73, which are under tension, coliapse the toggle 63, 65 to operate the .contact arm structures 25 to the .open position. When the toggle 63, 65 is collapsed, the tension springs 73 have not fully discharged and that'they are still under some tension operating against the knee pivot 67 to operate through the toggle .link 65 to .bias the tie bar 33 generally frontward or upward with the tie bar 33 being supported against frontward or upward movement by the engage.- ment of the front surfaces 129 (FIG. 5) of theinsulating hearing members 113 with the undersurfaces 127 ofthe supporting plates 125. During operation of the circuit breaker, the insu-. lating bearing members l14-remain' stationary with the pivot pin parts 105 of the steel core of the tie bar133 pivoting in the openings 1 15 of the bearing members 113. One of the bearing members 113 and part of the tie bar.33 are shownin the exploded position in FIG. 6. The assembly of the tie bar 33. in the breaker base 11 iseasyin that the bearings 113 are merely the base to be riveted over as shown in FIG. 5 in order to securely mount the'tie bar 33 in position for pivotal operating movement. The insulating bearings 113 provide the additional insulation to prevent a phase-to-phasefault or flashover from the one outer pole-unit contact arm structure (FIG. 3) through the one steelsupport plate 125, the steel core 101 of the tie bar 33, the other steel support plate 125m the other movable contact arm structure 25 of the other outer pole unit. For some applications, it is sufficient to provide an insulating bearing member 113 at only one end of the tie bar.33 and to pivotally mount the other endof thetie bar 33 in a suitable. opening in a suitable metal support plate 125. For other applications, it is-desirable to provide two of the insulating bearing members 113 for adequate insulation to protect against the phase-to-phase fault between the two outer pole-units through thesteel core 101. The.insulatingbearing.members 113 can be any suitable rigid insulating-material. For example, they can be made of a paper or cloth base resin-impregnated laminate: Bearing members 113 ofa phenolic resin cotton laminateconstructed in accordance with the above description have worked satisfactorily.

lclaim:

support means supporting said tiebar for pivotal movement about the elongated axis thereof, said. tie bar at at least one end thereof comprising a metal support pin; an insu-. lating bearing having an opening therein receiving said support pin for pivotal movement in said opening, a support surface on said housing engaging said insulating bearing to support said insulating bearing against backward movement in said housing, anda metalsupport plate over said insulating bearing secured to said housing to support said insulating bearing against frontward movement in said housing. t

2. A'multipole circuit interrupter according to claimxl, said metal bar comprising a shouldensurface at said .one end 6 thereof in proximity to said support pin, said insulating housing comprising asidewall having a slot therein, said support .pin ,being positioned in said slot with said insulating bearing ing having athickness front surface engaging'said thickness undersurface of said support plate to limit frontward movement of said insulating bearing.

4. A'multipole circuit interrupter according to claim 3 said insulating housing having a pair of openings in the back thereof, said support plate comprising a pair of legs extending through said openings and'being riveted over to fixedly support said support plate in said housing.

5. A multipole circuit interrupter according to claim 4, said insulatinghousing being a two-part housing comprising an insulating base and an insulating-cover secured to said insulating base, said slot being inth'esidewall of said insulating base and being-open at the front of said insulating base whereby said tie bar is mounted by beingdropped in said base with said insulating -bearingthereon during which movement said support pin moves down through the open front of said slot to the mounted position in said base. 7

6. A-multipole circuit interrupter according to claim 3, ans said-insulating bearing separating the periphery of said support-pin fromsaid undersurface of said support plate by a dimension greater than the diameter of said support pin.

7. Amultipole circuit interrupter according to claim 1, said tie barcomprising a separate metalsupport pin at each of the two oppositeends thereof, apair of said insulating bearings, each of 'said insulating bearings having an opening therein receiving a'differentone of said support pins, a pair of said support surfaces on said housing engaging said insulating bearings'to support said insulating bearings an tie bar against backward movement in said housing,*a pair of said metal support plates, each of said metal support plates being supported over a different one of said insulating bearings and being secured to said-housing whereby said metal support plates support said insulating bearings and tie bar against frontward movement in said housing.

8. A multipole circuit interrupter according to claim 7, said metal bar comprising a separate shoulder surface at each end thereof with each shoulder surface being in proximity to the associated support pin, said insulatinghousing comprising a pair of sidewalls with each sidewall having a slot therein, said support pins being positioned in said slots with each of said insulating bearings engaging the associated shoulder surface of said metal bar at one side thereof and the associated internal sidewallsurface of said housing adjacent the associated slot at the other side thereof whereby said insulating bearings support said tie bar against axial movement in said housing.

9. A multipole circuit interrupter according to claim 8, each of said support plates being a flat support plate having a thickness undersurface, each of said insulating bearings being a flat insulating bearing having a thickness front surface engaging the associated thickness undersurface of the associated support plate to limit frontward movement of the insulating bearing.

10. A multipole circuit interrupter'according to claim 9, said insulating housing being atwo-part housing comprising an insulating base-and an insulating cover secured to said insulatingbase, said slots beingin the sidewalls of said insulating base and being open at the front of said insulating base whereby said tie'bar is mounted by being dropped in said base with said insulatingbearings thereon during which movement said support pins move down through the open-front of said slots to the mounted position in said base, said insulating base'having a pair of openingsin the back thereof for each of said support plates, each of said support plates comprising a pair of legs exassociated support pin from the associated undersurface of the associated support plate by a dimension greater than the diameter of the associated support pin. 

1. A multipole circuit interrupter comprising an insulating housing, a circuit interrupter mechanism supported in said insulating housing, said circuit interrupter mechanism comprising a stationary contact for each pole, a movable contact arm structure for each pole cooperating with the associated stationary contact, an elongated tie bar common to all of said poles, said movable contact arm structures being supported on said tie bar for simultaneous movement with said tie bar, said tie bar comprising a rigid metal bar and an insulating cover on said metal bar, support means supporting said tie bar for pivotal movement about the elongated axis thereof, said tie bar at at least one end thereof comprising a metal support pin, an insulating bearing having an opening therein receiving said support pin for pivotal movement in said opening, a support surface on said housing engaging said insulating bearing to support said insulating bearing against backward movement in said housing, and a metal support plate over said insulating bearing secured to said housing to support said insulating bearing against frontward movement in said housing.
 2. A multipole circuit interrupter according to claim 1, said metal bar comprising a shoulder surface at said one end thereof in proximity to said support pin, said insulating housing comprising a sidewall having a slot therein, said support pin being positioned in said slot with said insulating bearing engaging said shoulder surface of said metal bar at one side thereof and the internal sidewall surface of said housing adjacent said slot at the other side thereof to support said tie bar against axial movement toward said sidewall.
 3. A multipole circuit interrupter according to claim 2, said support plate being a flat support plate having a thickness undersurface, said insulating bearing being a flat insulating bearing having a thickness front surface engaging said thickness undersurface of said support plate to limit frontward movement of said insulating bearing.
 4. A multipole circuit interrupter according to claim 3 said insulating housing having a pair of openings in the back thereof, said support plate comprising a pair of legs extending through said openings and being riveted over to fixedly support said support plate in said housing.
 5. A multipole circuit interrupter according to claim 4, said insulating housing being a two-part housing comprising an insulating base and an insulating cover secured to said insulating base, said slot being in the sidewall of said insulating base and being open at the front of said insulating base whereby said tie bar is mounted by being dropped in said base with said insulating bearing thereon during which movement said support pin moves down through the open front of said slot to the mounted position in said base.
 6. A multipole circuit interrupter according to claim 3, ans said insulating bearing separating the periphery of said support pin from said undersurface of said support plate by a dimension greater than the diameter of said support pin.
 7. A multipole circuit interrupter according to claim 1, said tie bar comprising a separate metal support pin at each of the two opposite ends thereof, a pair of said insulating bearings, each of said insulating bearings having an opening therein receiving a different one of said support pins, a pair of said support surfaces on said housing engaging said insulating bearings to support said insulating bearings an tie bar against backward movement in said housing, a pair of said metal support plates, each of said metal support plates being supported over a different one of said insulating bearings and being secured to said housing whereby said metal support plates support said insulating bearings and tie bar against frontward movement in said housing.
 8. A multipole circuit interrupter according to claim 7, said metal bar comprising a separate shoulder surface at each end thereof with each shoulder surface being in proximity to the associated support pin, said insulating housing comprising a pair of sidewalls with each sidewall having a slot therein, said support pins being positioned in said slots with each of said insulating bearings engaging the associated shoulder surface of said metal bar at one side thereof and the associated internal sidewall surface of said housing adjacent the associated slot at the other side thereof whereby said insulating bearings support said tie bar against axial movement in said housing.
 9. A multipole circuit interrupter according to claim 8, each of said support plates being a flat support plate having a thickness undersurface, each of said insulating bearings being a flat insulating bearing having a thickness front surface engaging the associated thickness undersurface of the associated support plate to limit frontward movement of the insulating bearing.
 10. A multipole circuit interrupter according to claim 9, said insulating housing being a two-part housing comprising an insulating base and an insulating cover secured to said insulating base, said slots being in the sidewalls of said insulating base and being open at the front of said insulating base whereby said tie bar is mounted by being dropped in said base with said insulating bearings thereon during which movement said support pins move down through the open front of said slots to the mounted position in said base, said insulating base having a pair of openings in the back thereof for each of said support plates, each of said support plates comprising a pair of legs extending through the associated openings and being riveted over to fixedly support the support plate in said housing, and each of said insulating bearings separating the periphery of the associated support pin from the associated undersurface of the associated support plate by a dimension greater than the diameter of the associated support pin. 